The use of backscattered light in fiber optic cables has found increasing acceptance in a variety of applications. Because light can be backscattered from any location along the length of a fiber, information can be obtained over significant distances and such systems are often referred to as “distributed” sensors.
Because distortion or deformation of the fiber can be sensed, distributed sensors comprised of fiber optic cable can be used to sense temperature, pressure, strain, acoustic events, and the like. Distributed systems have been used advantageously in oilfield applications, in traffic monitoring, and in military/security applications, among others. In particular, distributed acoustic sensing (DAS) systems are finding increased usage for sensing seismic events, i.e. acoustic signals that have been transmitted at least partly along a path through the earth.
In a typical fiber optic-based distributed sensing system, one or more fiber optic cables designed to collect distributed strain or acoustic measurements are deployed in a desired location and coupled to the sensing subject by suitable means. In oilfield applications, the cables may be distributed in one or more boreholes, in or on the surface of the earth, and/or in or on a seafloor. One or more light boxes containing laser light sources and signal-receiving means are optically coupled to the fiber. In some embodiments, the light source may be a long coherence length phase-stable laser that is used to transmit direct sequence spread spectrum encoded light down the fiber. The cable may be double-ended, i.e. may be bent in the middle so that both ends of the cable are at the source, or it may be single-ended, with one end at the source and the other end at a point that is remote from the source. The length of the cable can range from a few meters to several kilometers, or even hundreds of kilometers. In any case, measurements can be based solely on backscattered light, if there is a light-receiving means only at the source end of the cable, or a light receiving means can be provided at the second end of the cable, so that the intensity of light received at the second end of the fiber optic cable can also be measured.
When it is desired to make measurements, the light source transmits at least one light pulse into the end of the fiber optic cable and a backscattered signal is received at the signal-receiving means. Localized strain or other disruptions cause small changes to the fiber, which in turn produce changes in the backscattered light signal. The returning light signal thus contains both information about the deformation of the fiber and location information indicating where along the fiber it occurred. Known optical time-domain reflectometry (OTDR) methods can be used to infer information about the sensing subject based on the backscattered signal from one or more segments of the fiber adjacent to the subject. In some instances, the location of the backscattering reflection at a point along the fiber is determined using spread spectrum encoding, which uniquely encodes the time of flight along the length of the fiber, dividing the fiber into discrete channels along its length.
Because of the nature of backscattering measurements, deformations that affect the distance between the back-scatterer and the light source, i.e. axial deformations, are much more detectable than lateral deformations. This in turn reduces their utility. It is therefore desirable to provide a DAS system in which sensitivity to lateral, or “cross-axial” or “broadside,” signals is improved. It is further desirable to provide a DAS system that has improved directivity, or azimuthal anisotropy.